The measurement of potential differences in a sample is a prerequisite, inter alia, to the calculation of the electrical conductivity of the sample or a region or cross section of the sample. For this purpose the sample is generally excited with an electrical current. From the measured voltage drop and the amplitude of the current flow, the electrical resistance and its inverse, the conductivity of the sample can be calculated.
Various measuring units have been used in the art for the determination of resistivity or conductivity and methods of measuring the electrical conductivity are known.
Basically these units comprise at least one electrode pair. The so-called Kohlrausch arrangement encompasses two mutually opposite solid body electrodes, for example of carbon or platinum or platinized metal. The form of these electrodes can vary.
For carrying out the electrical conductivity measurement an alternating current voltage is applied to the electrode and current is thereby fed to an electrically conductive sample via electrodes. The resulting electrode potential is measured by the same electrodes as supplied the current.
A simple embodiment for carrying out such measurement uses so-called pole-to-pole measurements which are practiced with two electrodes. With these electrodes the current and the potential difference between the two electrodes are measured.
A drawback of this system is that the measurement results are influenced by the polarization effect at the boundary layers between the electrodes of the sample. In special cases with ground samples and other heterogeneously structured samples there arises between the sample and the electric current feeding electrodes high resistances which are functions of the point at which the electric current is fed to the sample.
A four-point method (four electrode method) of measuring the potential in soil samples can also be practiced. In that case, two electrodes are used to feed electrical signals into the sample. The electrodes serve at any given point in time either for applying the excitation alternating current or for the high ohmic measurement of the potential. From the known amplitude of the current flow and the measured potential difference, the resistance or the conductivity of the sample can be readily calculated. The transition resistance and other potential drops due to polarization effects at the interface between the sample and the electrode are not determined with such measurements.
A drawback of the prior art system for simultaneous measurement of the potential difference and application of electric current to the sample at two closely adjoining electrodes or two surfaces of the same electrode which closely adjoin one another is that the electrode must have a complex and expensive construction. With heterogeneously structured and dense samples, for example, soils, aquifers, rock and ore strata, a large number of electrodes must be employed in such samples. For measurements which are to determine the conductivity of specific areas and/or tomographic measurements which are to determine conductivity or resistivity of sections through the sample, a large number of different current-supply and measurement electrodes must be provided with different current-supply and measurement configurations. The measurement arrangements for complex samples especially those utilized for geophysical or geoelectrical analyses are known from German patent document 198 37 828.
There are, however, different drawbacks of such systems depending upon the measurement method. In principle, of course, for all measurements which are to be carried out utilizing a common electrode for application of the electric current and measurement of the potential, for example in the two electrode method, measurement errors arise because of the high variable and unknown transition resistances between the electrode and the sample, especially when the electrode is driven into the ground.
Four electrode measurement systems are either expensive or complex to use because installation becomes a major factor. From each measurement position, an additional electrode must be employed and the distances from each electrode to another electrode may not be precisely defined.
For area-like and tomographic measurements utilizing sequential feed of electric current through a sample, measurement errors occur as a result of polarization effects at the electrodes since in these methods the potential measurements are effected at the moment that the current is applied at the very same electrode surfaces and thus with polarized electrodes.